Field effect transistor with self-aligned source and heavy body regions

ABSTRACT

A field effect transistor includes a plurality of trenches extending into a semiconductor region of a first conductivity type. The plurality of trenches includes a plurality of gated trenches and a plurality of non-gated trenches. A body region of a second conductivity extends in the semiconductor region between adjacent trenches. A dielectric material fills a bottom portion of each of the gated and non-gated trenches. A gate electrode is disposed in each gated trench. A conductive material of the second conductivity type is disposed in each non-gated trench such that the conductive material and contacts corresponding body regions along sidewalls of the non-gated trench.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/152,041, filed Jun. 2, 2011, which is a continuation of U.S.application Ser. No. 12/822,008, filed Jun. 23, 2010, now U.S. Pat. No.7,955,920, which is a continuation of U.S. application Ser. No.12/582,487, filed Oct. 20, 2009, now U.S. Pat. No. 7,767,524, which is acontinuation of U.S. application Ser. No. 12/418,949, filed Apr. 6,2009, now U.S. Pat. No. 7,625,799, which is a continuation of U.S.application Ser. No. 12/125,242, filed May 22, 2008, now U.S. Pat. No.7,514,322, which is a continuation of U.S. application Ser. No.11/450,903, filed Jun. 8, 2006, now U.S. Pat. No. 7,393,749, whichclaims the benefit of U.S. Provisional Application No. 60/689,229, filedJun. 10, 2005, all of which are incorporated herein by reference intheir entirety for all purposes.

This application is also related to U.S. application Ser. No.11/026,276, filed Dec. 29, 2004, and U.S. application Ser. No.11/441,386, filed on May 24, 2006, both of which are incorporated byreference in their entirety for all purposes.

BACKGROUND

The invention relates to semiconductor power device technology, and moreparticularly to charge balance field effect transistors and methods ofmanufacturing same.

The development of device structures for high current switches has seenprogress from planar gate vertical DMOS to trench gate structuresincluding those with shield electrodes. Early development projectsfocused on reducing the specific on-state resistance, R_(SP). Later,other performance attributes such as gate charge (the charge required toturn the device on and off) were added to the development objectives.More recently, these merit features have evolved into specific uniqueobjectives depending on the specific application for the switch.

Because of its influence on the switching speed of the MOSFET, theproduct of the specific on-resistance and the gate-drain charge,R_(SP)×Q_(GD), is referred to as the figure-of-merit (FOM) for the topswitch in synchronous buck converters which are ubiquitous in manyelectronic systems. In like fashion, the low side MOSFET whose powerdissipation depends on conduction losses, is judged based on a FOMdepending on the total gate charge, R_(SP)×Q_(G). Shielded gatestructures can significantly improve both of these figures-of-merit. Inaddition, by increasing the depth of the shield electrode, chargebalance can be improved which allows higher than parallel planebreakdown for a given drift region concentration, thus reducing R_(SP).

Implementing such a charge balance device structure for low voltageMOSFET has proved difficult because of process and material variationsresulting in an imbalance in the carrier types which in turn causereduced breakdown voltage. Assuming charge balance results in a flatelectric field in the drift region, it can be shown that the product ofthe doping concentration N, and the width of the drift region columns W,must be less than the product of the semiconductor permittivity and thecritical electric field divided by the electron charge q:

${N \cdot W} < \frac{ɛ_{S} \cdot E_{C}}{q}$

Consequently, a lower BV_(DSS) target requires greater dopingconcentration so that the drift region column width must decrease tomaintain charge balance. For example, a 30V device with about 2×10¹⁶cm⁻³ drift region concentration requires a mesa width less that about1.4 μm for optimum charge balance. This condition however does notresult in an improvement in the R_(SP) since 2×10¹⁶ cm⁻³ can support 30Vwithout charge balance. If the concentration is doubled to reduce driftregion resistance, the required mesa width is halved to about 0.7 μm.These fine dimensions are difficult to achieve considering all thefeatures that must fit within the cell architecture such as the heavybody junction needed for avalanche ruggedness.

In most charge balance architectures, the drift region is an n-typeregion on a heavily doped n-type substrate. In some variations, thetrench sidewalls are implanted with boron to provide opposite polaritycharge. For low voltage devices, each of these methods may suffer fromprocess variations that result in charge imbalance and a relatively widedistribution in the performance features including R_(SP), Q_(GD), andBV_(DSS). The process variations come from several sources includingepitaxial layer concentration, gate electrode depth relative to thep-well depth, mesa width, and shield dielectric thickness.

Thus there is a need for improved charge balance MOSFET cell structuresand methods of manufacture.

BRIEF SUMMARY

In accordance with an embodiment of the invention, a field effecttransistor is formed as follows. A semiconductor region of a firstconductivity type with an epitaxial layer of a second conductivityextending over the semiconductor region is provided. A trench extendingthrough the epitaxial layer and terminating in the semiconductor regionis formed. A two-pass angled implant of dopants of the firstconductivity type is carried out to thereby form a region of firstconductivity type along the trench sidewalls. A threshold voltage adjustimplant of dopants of the second conductivity type is carried out tothereby convert a conductivity type of a portion of the region of firstconductivity type extending along upper sidewalls of the trench to thesecond conductivity type. Source regions of the first conductivity typeflanking each side of the trench are formed.

In accordance with another embodiment of the invention, a MOSFETincludes a trench extending into a semiconductor region and a shieldelectrode in a lower portion of the trench. The shield electrode isinsulated from the semiconductor region by a shield dielectric. A gateelectrode is in an upper portion of the trench over but insulated fromthe shield electrode. The semiconductor region includes a substrate of afirst conductivity type and a first silicon region of a secondconductivity type over the substrate. The first silicon region has afirst portion extending to a depth intermediate a top surface and abottom surface of the gate electrode. The first silicon region has asecond portion extending to a depth intermediate a top surface and abottom surface of the shield electrode. The semiconductor region furtherincludes a second silicon region of the first conductivity type betweenthe trench and the second portion of the first silicon region. Thesecond silicon region has a laterally-graded doping concentrationdecreasing in a direction away from the trench sidewalls. Thesemiconductor region also includes a source region of the firstconductivity type in the first silicon region adjacent the trench.

In accordance with another embodiment of the invention, a FET is formedas follows. A semiconductor region of a first conductivity type with anepitaxial layer of a second conductivity extending over thesemiconductor region is provided. A first silicon etch is performed toform an upper trench portion extending into and terminating within theepitaxial layer. A protective material is formed extending alongsidewalls of the upper trench portion and over mesa regions adjacent theupper trench portion but not along a bottom surface of the upper trenchportion. A second silicon etch is performed to form a lower trenchportion extending from the bottom surface of the upper trench portionthrough the epitaxial layer and terminating within the semiconductorregion. The lower trench portion is narrower than the upper trenchportion. A two-pass angled implant of dopants of the first conductivitytype is performed to form a silicon region of first conductivity typealong sidewalls of the lower trench portion. The protective materialblocks the implant dopants from entering the sidewalls of the uppertrench portion and the mesa region adjacent the upper trench portion.

In accordance with yet another embodiment of the invention, a MOSFETincludes a trench extending into a semiconductor region. The trench hasa lower portion and an upper portion, the lower portion being narrowerthan the upper portion. The MOSFET further includes a shield electrodein the lower portion of the trench, the shield electrode being insulatedfrom the semiconductor region by a shield dielectric. A gate electrodeis in the upper portion of the trench, the gate electrode being over butinsulated from the shield electrode. The semiconductor region includes asubstrate of a first conductivity type and a first silicon region of asecond conductivity type over the substrate. The first silicon regionhas a first portion extending to a depth intermediate a top surface anda bottom surface of the gate electrode. The first silicon region has asecond portion extending to a depth intermediate a top surface and abottom surface of the shield electrode. The semiconductor region furtherincludes a second silicon region of the first conductivity type betweenthe lower trench portion and the second portion of the first siliconregion. The second silicon region has a laterally-graded dopingconcentration decreasing in a direction away from the sidewalls of thelower trench portion. A source region of the first conductivity type isin the first silicon region, the source region being adjacent the uppertrench portion.

In accordance with another embodiment of the invention, a MOSFET isformed as follows. A substrate of a first conductivity type with anepitaxial layer of a first conductivity extending over the substrate isprovided. A first silicon etch is performed to form an upper trenchportion extending into and terminating within the epitaxial layer. Aprotective material is formed extending along sidewalls of the uppertrench portion and over mesa regions adjacent the upper trench portionbut not along a bottom surface of the upper trench portion. A secondsilicon etch is performed to form a lower trench portion extending fromthe bottom surface of the upper trench portion through the epitaxiallayer and terminating within the substrate, the lower trench portionbeing narrower than the upper trench portion. A two-pass angled implantof dopants of the first conductivity type is carried out to form asilicon region of first conductivity type along sidewalls of the lowertrench portion, the protective material blocking the implant dopantsfrom entering the sidewalls of the upper trench portion and the mesaregion adjacent the upper trench portion. A shield dielectric liningsidewalls and bottom surface of the lower trench portion I formed. Ashield electrode is formed in the lower trench portion. A gatedielectric layer is formed along sidewalls of the upper trench portion.A gate electrode is formed in the upper trench portion over butinsulated from the shield electrode.

In accordance with another embodiment of the invention, a MOSFETincludes a trench extending into a semiconductor region. The trench hasa lower portion and an upper portion, the lower portion being narrowerthan the upper portion. The MOSFET further includes a shield electrodein the lower portion of the trench, the shield electrode being insulatedfrom the semiconductor region by a shield dielectric. A gate electrodeis in the upper portion of the trench, the gate electrode being over butinsulated from the shield electrode. The semiconductor region includes asubstrate of a first conductivity type, an epitaxial layer of the firstconductivity type over the substrate, and a body region of a secondconductivity type in the epitaxial layer. A source region of the firstconductivity type is in the body region, the source region and aninterface between the body region and the substrate defining a channelregion. A silicon region of the first conductivity type extends alongsidewalls of the lower portion of the trench and into a lower portion ofthe channel region. The silicon region has a laterally-graded dopingconcentration decreasing in a direction away from the sidewalls of thetrench.

In accordance with another embodiment of the invention, a MOSFET isformed as follows. A substrate of a first conductivity type with anepitaxial layer of the first conductivity type extending over thesubstrate is provided. A gate trench is formed extending through theepitaxial layer and terminating within the substrate. A shielddielectric lining sidewalls and bottom surface of the gate trench isformed. A shield electrode is formed in the gate trench. A gatedielectric layer is formed along upper sidewalls of the gate trench. Agate electrode is formed in the gate trench, the gate electrode beingover but insulated from the shield electrode. A deep dimple is formedextending through the epitaxial layer and terminating within thesubstrate, the deep dimple being laterally spaced from the gate trench.The deep dimple is filled with silicon material of the secondconductivity type.

In accordance with another embodiment of the invention, a MOSFETincludes a substrate of a first conductivity type and an epitaxial layerof the first conductivity type over the substrate. A gate trench extendsthrough the epitaxial layer and terminates within the substrate. Ashield dielectric lines sidewalls and bottom surface of the gate trench.A shield electrode is in a lower portion of the gate trench. A gatedielectric layer extends along upper sidewalls of the gate trench. Agate electrode is in the gate trench, the gate electrode being over butinsulated from the shield electrode. A deep dimple extends through theepitaxial layer and terminates within the substrate, the deep dimplebeing laterally spaced from the gate trench. The deep dimple is filledwith silicon material of the second conductivity type.

In accordance with another embodiment of the invention, a MOSFET isformed as follows. A substrate of a first conductivity type is provided.An epitaxial layer of the first conductivity type is formed over thesubstrate. A plurality of gate trenches extending through the epitaxiallayer and terminating within the substrate is formed. A shielddielectric lining sidewalls and bottom surface of each gate trench isformed. A shield electrode is formed in each gate trench. A gatedielectric layer is formed along upper sidewalls of each gate trench. Agate electrode is formed in each gate trench, the gate electrode beingover but insulated from the shield electrode. A plurality of ionimplantations of dopants of the second conductivity type into mesaregions between adjacent gate trenches are carried out to thereby form aplurality of pillars of second conductivity type extending through theepitaxial layer and terminating within the substrate, each pillar ofsecond conductivity type being located between every two gate trenches.

In accordance with another embodiment of the invention, a FET is formedas follows. A plurality of trenches are formed in a semiconductor regionof a first conductivity type, the plurality of trenches including aplurality of gated trenches and a plurality of non-gated trenches. Abody region of a second conductivity is formed in the semiconductorregion between adjacent trenches. A bottom portion of each of the gatedand non-gated trenches is filled with dielectric material. A gateelectrode is formed in each gated trench over the dielectric material. Aconductive material of the second conductivity type is formed in eachnon-gated trench over the dielectric material such that the conductivematerial contacts the body region along sidewalls of each non-gatedtrench.

In accordance with another embodiment of the invention, a FET is formedas follows. A plurality of trenches are formed in a semiconductor regionof a first conductivity type, the plurality of trenches including aplurality of gated trenches and a plurality of non-gated trenches. Ashield electrode is formed in a bottom portion of each gated andnon-gated trench. A body region of a second conductivity is formed inthe semiconductor region between adjacent trenches. A dielectric layeris formed over the shield electrode in each non-gated trench. Aconductive material of the second conductivity type is formed in eachnon-gated trench over the dielectric layer such that the conductivematerial contacts body regions along sidewalls of the non-gated trench.

In accordance with another embodiment of the invention, a FET is formedas follows. A plurality of trenches are formed in a semiconductor regionof a first conductivity type, the plurality of trenches comprising aplurality of gated trenches and a plurality of non-gated trenches. Abody region of a second conductivity is formed in the semiconductorregion between adjacent trenches. A shield electrode is formed in abottom portion of each gated trench. A shield electrode is formed ineach non-gated trench, the shield electrode in each non-gated trenchhaving a top surface above a bottom surface of the body region. Aconductive material of the second conductivity type is formed in eachnon-gated trench such that the conductive material contacts body regionsalong sidewalls of the non-gated trench, the conductive material alsobeing in contact with the shield electrode in each non-gated trench.

In accordance with another embodiment of the invention, a FET is formedas follows. A plurality of trenches are formed in a semiconductor regionof a first conductivity type, the plurality of trenches including aplurality of gated trenches and a plurality of non-gated trenches. Ashield electrode is formed in a bottom portion of each gated andnon-gated trench. A body region of a second conductivity is formed inthe semiconductor region between adjacent trenches. A dielectric layeris formed over the shield electrode in each non-gated trench. A two-passangled implant of dopants of the second conductivity type into exposedupper sidewalls of each non-gated trench is carried out to thereby forma heavy body region in each body region.

In accordance with another embodiment of the invention, a FET is formedas follows. A plurality of trenches are formed in a semiconductor regionof a first conductivity type, the plurality of trenches comprising aplurality of gated trenches and a plurality of non-gated trenches. Abody region of a second conductivity is formed in the semiconductorregion between adjacent trenches. A bottom portion of each of the gatedand non-gated trenches is filled with dielectric material. A gateelectrode is formed over the dielectric material in each gated trench. Atwo-pass angled implant of dopants of the second conductivity type intoexposed upper sidewalls of each non-gated trench is carried out tothereby form a heavy body region in each body region.

The following detailed description and the accompanying drawings providea better understanding of the nature and advantages of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D are simplified cross section views depicting an exemplaryprocess flow for forming an n-channel charge balance MOSFET using ap-type epitaxial layer according to one embodiment of the presentinvention;

FIGS. 2A-2E are simplified cross section views depicting an exemplaryprocess flow for forming an n-channel charge balance MOSFET using ap-type epitaxial layer according to another embodiment of the presentinvention;

FIGS. 3A-3E are simplified cross section views depicting an exemplaryprocess flow for forming an n-channel charge balance MOSFET using an-type epitaxial layer according to another embodiment of the presentinvention;

FIG. 4 is a simplified exemplary cross section view showing a chargebalance shielded gate MOSFET with a silicon-filled trench according toan embodiment of the present invention;

FIGS. 5A-5B are simplified cross section views depicting an exemplaryprocess flow for forming a charge balance MOSFET using multiple ionimplantation steps according to an embodiment of the present invention;

FIGS. 6A-6G are simplified cross section views illustrating an exemplaryprocess flow for forming a trench gate FET with self-aligned non-gatedtrenches incorporated between the gated trenches, according to anembodiment of the present invention;

FIGS. 7A-7H are simplified cross section views illustrating anotherexemplary process flow for forming a shielded gate FET with self-alignednon-gated trenches incorporated between the gated trenches, according toan embodiment of the present invention;

FIGS. 8A-8H are simplified cross section views illustrating yet anotherexemplary process flow for forming a shielded gate FET with self-alignednon-gated trenches incorporated between the gated trenches, according toanother embodiment of the present invention;

FIG. 9 is a simplified cross section view of a shielded gate FET with anon-gated trench wherein the heavy body regions are formed in the bodyregions rather than inside the non-gated trench; and

FIG. 10 is a simplified cross section view of a trench gate FET with anon-gated trench wherein the heavy body regions are formed in the bodyregions rather than inside the non-gated trench.

DETAILED DESCRIPTION

In accordance with an embodiment of the present invention, an n-channelshielded gate MOSFET, which is particularly useful for low voltageapplications but not limited thereto, is formed in a p-type epitaxiallayer rather than the conventional n-type epitaxial layer. This providesopportunities to simplify the process, such as eliminating the processsteps associated with forming the p-type body region.

FIGS. 1A-1D are simplified cross section views illustrating a processsequence for forming a charge balance MOSFET according to an embodimentof the present invention. In FIG. 1A, a p-type epitaxial layer 44 isformed (e.g., by selective epitaxial growth of silicon) over a siliconsubstrate 42. In one embodiment, the starting wafer material includessubstrate 42 and its overlying p-type epitaxial layer 44. A conventiontrench etch is carried out to form trench 46 extending through epitaxiallayer 44 and terminate is substrate 42. An optional anneal step may thenbe performed to repair damaged silicon and to round the trench corners.

In FIG. 1B, a two-pass angled implant 50 is carried out to form ann-type region 48 along the trench sidewalls and bottom using knowntechniques. While not shown, the mesa regions are blocked from receivingthe implant dopants. An optional diffuse and drive step may be used todrive the implanted ions further into the silicon. In FIG. 1C, a shielddielectric 53 and shield electrode 54 are formed in the lower portion oftrench 46 using conventional techniques. An inter-poly dielectric (IPD)layer 56 is then formed over shield electrode 54. Using known methods, agate dielectric 53 lining the upper trench sidewalls is formed followedby a recessed gate electrode 58 formed over IPD layer 56.

In FIG. 1D, a threshold voltage (Vt) adjust implant of p-type dopants iscarried out to form p-type regions 62 using known techniques. The dopingconcentration of the Vt implant is selected so that the implantcounter-dopes the portion of n-type region 48 extending along thechannel region, and the desired doping concentration is obtained in thechannel region of the transistor. A conventional source implant is thenperformed to form n+ source regions 64. The thermal budget for thesource diffusion also serves to drive in the Vt adjust implant. Heavybody regions 66 are then formed using conventional techniques. As seenin FIG. 1D, a large portion of p-type epitaxial layer 44 still remainsp-doped. To complete the device, a dielectric layer 68 such asborophosphosilicate glass (BPSG) is deposited and patterned to covertrench 46 and a portion of source regions 64. A source interconnectlayer 70 (e.g., comprising metal) is then formed over the structure toelectrically contact source regions 64 and heavy body regions 66.

During the heat cycles associated with the above steps, n-type dopantsin both the n-type region 48 and substrate 42 diffuse out. As a result,the doping concentration in the out-diffused n-type region 48 isgreatest near the trench and gradually decreases in the direction awayfrom trench sidewalls. Similarly, the out-diffusion of dopants fromsubstrate 42 into epitaxial layer 44 leads to formation of a gradedn-type region with a doping concentration which gradually decreases inthe direction from the original interface between substrate 42 andepitaxial layer 44 (shown as a dotted line in FIGS. 1C and 1D) towardthe top surface. This effectively moves the boundary between substrate42 and epitaxial layer 44 upwards.

In FIG. 1D, the portion of n-type region 48 extending below thetransistor channel regions together with the portions of p-typeepitaxial layer 44 directly adjacent these portions of n-type region 48form columns of a charge balance structure. As can be seen from theprocess depicted by FIGS. 1A-1D, these p-type and n-type columns of thecharge balance structure are advantageously formed in a self-alignedmanner. The charge balance structure together with the shielded gatestructure reduces both the gate-to-drain charge Qgd and theon-resistance, and increases the breakdown voltage. These improvementsare achieved using a simple process wherein the process steps forforming the well region (also referred to as body region) areeliminated. In one embodiment, the conductivity type of the variousregions is reversed so that a p-channel MOSFET is obtained. In anotherembodiment, substrate 42 comprises multiple layers of the sameconductivity type silicon with different doping concentrations.

In one variation of the FIGS. 1A-1D embodiment, a very lightly dopedp-type epitaxial layer is used, and subsequently, a two-pass angledimplant of p-type dopants is carried out to form a p-type region alongthe trench sidewalls. Next, a two-pass angled implant of n-type dopantsis carried out to form a n-type region along trench sidewalls. Thedoping concentration, implant energy and other implant parameters can beproperly selected to ensure that the p-type region laterally extendsfurther than the n-type region, so that the p-type and n-type regionsform the two columns of the charge balance structure. Thus, since boththe p-type and n-type columns in the charge balance structure are dopedusing the implantation steps, any charge imbalance resulting from dopingvariation in the epitaxial layer can be eliminated.

Thus, by carefully optimizing the p-type body and the n-type sidewallimplant and drive-in conditions, charge balance and gate overlap of thep-body are greatly enhanced compared to conventional techniques. As aresult, lower specific on-state resistance and much lower gate-draincharge are achieved. Simulations of exemplary structures indicate atleast 10-20% lower RSP and half the gate-drain charge compared toconventional shielded gate structures.

In an alternate method, a shallow trench is etched and an oxide layerand then a nitride layer are formed to protect the mesa and trenchsidewalls from a later deep trench etch. With the nitride remaining onthe sidewalls of the shallow trench, the deeper trench sidewalls areexposed for an angled implant. This confines the implant to the lowerportions of the epitaxial region and out of the channel region, allowingthe p-type epitaxial layer to act as the channel and as the deepjunction for charge balance purposes. An exemplary process flow forobtaining such a structure is illustrated in FIGS. 2A-2D.

In FIG. 2A, a p-type epitaxial layer 82 is formed (e.g., by selectiveepitaxial growth of silicon) over a highly doped n-type substrate 80. Atrench 84 is then etched to an intermediate depth within epitaxial layer82. A first dielectric layer 86 (e.g., comprising oxide) is then formedlining the sidewalls and bottom of trench 84 and extending on top of thesilicon mesa adjacent trench 84. As will be seen, it is desirable toform the first dielectric layer 86 such that the portion of the firstdielectric layer 86 extending over the silicon mesa is thicker than theportion of the first dielectric layer 86 inside trench 84. One way toobtain a thicker dielectric over the mesa region is to form a compositelayer of, for example, ONO, in a similar manner to that depicted inFIGS. 13A-13L of the commonly assigned U.S. patent application Ser. No.11/441,386, filed on May 24, 2006, which application is incorporatedherein by reference in its entirety. Using known techniques, a seconddielectric layer (e.g., comprising nitride) is then formed over firstdielectric layer 86 and then etched to form dielectric (e.g., nitride)spacers 87.

In FIG. 2B, with dielectric spacers 87 serving as a protective spacers,the exposed portions of the first dielectric layer 86 are etched untilepitaxial layer 82 becomes exposed along the trench bottom. Given thatthe first dielectric layer 86 is formed to have a greater thickness overthe mesa region than along the trench bottom, the mesa surface remainscovered by the first dielectric layer (albeit thinner) after the etch.

In FIG. 2C, a further silicon etch is carried out whereby the exposedbottom surface of trench 84 is extended clear through epitaxial layer 82and into substrate 80 to form a deeper trench 85. Trench 85 thus has anarrower lower portion than its upper portion. With the first dielectriclayer 86 and dielectric spacers 87 serving to protect the mesa surfaceand the upper trench sidewalls, a two-pass angled implant 83 of n-typedopants is carried out to form n-type silicon region 88 along theexposed lower sidewalls of trench 85. As shown n-type layer 88 mergeswith substrate 80. Dielectric spacers 87 prevent the implant fromgetting into the channel region.

In FIG. 2E, dielectric spacers 87 and first dielectric layer 86 areremoved using conventional techniques. A shield dielectric 89 and shieldelectrode 90 are then formed in the lower portion of trench 85 usingconventional techniques. An inter-poly dielectric (IPD) layer 92 isformed over shield electrode 90 using known methods. Gate dielectric 96and gate electrode 94 are then formed over IPD layer 92 usingconventional techniques. Source regions 93 and heavy body regions 95 areformed using known techniques. A dielectric layer 97 such as BPSG isthen deposited over the top of the structure and patterned to cover gate94 and a portion of source regions 93, and then a source interconnectlayer (not shown) is formed to electrical contact source regions 93 andheavy body regions 95.

A similar process to that illustrated by FIGS. 2A-2E can be carried outto form a shielded gate structure in an n-type epitaxial layer ratherthan a p-type epitaxial layer. The two-pass angled implant of n-typedopants suppresses the body diffusion into a bottom portion of thechannel region, which advantageously reduces the channel resistance.This implant also helps alleviate the high electric fields seen at thetrench sidewall. An exemplary process flow for forming such a structureis depicted by FIGS. 3A-3E. In FIG. 3A, an n-type epitaxial layer 402 isformed over an n-type substrate 400 using, for example, selectiveepitaxial growth. All subsequent steps leading to the formation of theshielded gate structure in FIG. 3E are similar to corresponding steps inFIGS. 2A-2E except that in FIG. 3E, prior to forming source regions 413and heavy body regions 415, a body implant of p-type dopants is carriedout to form body region 418. As shown by FIGS. 3D and 3E, silicon region408 formed by the two-pass angled implant diffuses up into the channelregion, thus reducing the channel resistance.

In accordance with another embodiment of the invention, a charge balanceshielded gate MOSFET is formed using a n-type epitaxial layer and a deepdimple that is filled in with epitaxially grown p-type silicon. Thisembodiment will be described using the exemplary cross section view inFIG. 4. In FIG. 4, between every two adjacent gated trenches 131, a deepdimple 133 extends through body region 136 and n-type epitaxial layer132 and terminating in the highly doped n-type substrate 130. Dimple 133is filled with p-type silicon material 134. The doping concentration ofn-type epitaxial layer 132 and the silicon material 134 in dimples 133are selected so that charge balance is obtained between these tworegions. The gated trench structure is otherwise similar to those in theprevious embodiments, and thus will not be described.

An exemplary method for forming the structure in FIG. 4 is as follows.An n-type epitaxial layer 132 is formed (e.g., by selective epitaxialgrowth) over highly doped n-type substrate 130. Body regions 136 ofp-type conductivity are formed by implanting dopants into epitaxiallayer 132. Body regions 136 extend to a depth sufficient to enableformation of channel regions. A subsequent silicon etch is carried outto form deep dimples 133 extending through body regions 136 andterminating in substrate 130. A selective epitaxial growth process isthen performed to fill deep dimples 133 with p-type silicon 134. Thegate trench 131 and the various materials therein, as well as sourceregions 140, heavy body regions 138 and other structural features areformed in accordance with known techniques. In one embodiment, the gatetrench and the gate and shield electrodes are formed before forming thedeep dimples. By extending dimples 133 below the substrate-epitaxiallayer interface, the high electric fields at the bottom of the pillarsare advantageously relieved. This allows for a thinner n-type epitaxiallayer, further reducing the on-state resistance.

FIGS. 5A and 5B show an alternative method of forming the deep p-typeregions 134 in FIG. 4 As shown in FIGS. 5A and 5B, p-type pillars 164are formed by implanting multiple high energy implants 172 of p-typedopants through a shallow dimple 168 into an n-type epitaxial layer 162.As shown, the dimple depth is slightly deeper than that of sourceregions 166. The dimple depth sets the reference point for the depth ofp-type pillars 164 since implants 172 are into the bottom surface ofdimple 168. The dose and energy of the implants 172 can be tuned toobtain the required doping profile in the p-type pillars 164. Sincethere is very little diffusion at the end of the process, the dopingprofiles of both the resulting p-type pillars 164 and n-type epitaxiallayer 162 are relatively flat. This results in improved processsensitivity.

In accordance with other embodiments of the invention, additionalmethods and structures for a charge balanced MOSFET (particularly usedfor low voltage applications though not limited thereto) use non-gatedshield trenches between gated trenches. These embodiments are describednext.

Charge balance trench gate FETs rely on the mesa width and the dopingconcentration of the drift region (typically an epitaxial layer) tocontrol the depletion under high reverse drain-source bias in order toobtain a higher breakdown than conventional trench gate FETs. The mesawidth is limited by the capabilities of the photolithography to define acontinuous heavy body contact region in the center of the mesa betweenadjacent gate trenches. However, in accordance with an embodiment of theinvention, the use of additional non-gated shield trenches interspersedbetween the gate trenches enables lowering the drift region resistivityfor the same breakdown voltage, effectively reducing the on-state of thedevice and allowing for improved charge balancing properties.

FIGS. 6A-6G are simplified cross section views illustrating an exemplaryprocess flow for forming a trench gate FET with self-aligned non-gatedtrenches incorporated between the gated trenches, according to anembodiment of the present invention. In FIG. 6A, using conventionaltechniques, trenches 202 and 204 are etched into silicon region 200. Inone embodiment, silicon region 200 comprises a highly doped n-typesubstrate and an n-type epitaxial layer over the substrate.

Trench 202 will be referred to as non-gated trench, and trench 204 willbe referred to as gated trench. A dielectric layer 206 (e.g., grownoxide) extending over mesa surfaces 208 and lining the sidewalls andbottom surfaces of trenches 202 and 204 is formed using knowntechniques. In FIG. 6B, a dielectric material 210 (e.g., deposited filmsuch as SACVD) filling the trenches and extending over the mesa regionsis deposited, using conventional methods. In FIG. 6C, a planarizationprocess is carried out such that a top surface of the dielectricmaterial 210 remaining in the trenches is substantially co-planar withmesa surfaces 208, using known techniques.

In FIG. 6D, using conventional methods, a masking layer (e.g.,photoresist) is deposited and patterned to form masking region 214covering non-gated trench 202, and then dielectric layer 206 anddielectric material 210 in gated trench 204 are recessed to thereby formthick bottom dielectric (TBD) 212 along the bottom of gated trench 204.In FIG. 6E, masking region 214 is removed and a gate dielectric layer220 (e.g., comprising oxide) lining sidewalls of gated trench 204 andextending over mesa surfaces and non-gated trench 202 is formed usingconventional techniques. A polysilicon layer is then deposited andrecessed into gated trench 204 to form recessed gate electrode 222 ingated trench 204. Conventional blanket body and source implantations arecarried out in the active region of the device to sequentially formp-type body regions 226 in silicon region 200 and then form highly dopedn-type source regions 224 in body regions 226.

In FIG. 6F, using known techniques, a dielectric layer (e.g., comprisingBPSG) is formed over the structure and then patterned and etched to formdielectric cap 230 extending only over gated trench 204. The samedielectric etch may be used to recess dielectric materials 206 and 210in non-gated trench 202 sufficiently to partially expose sidewalls ofbody regions 226. Dielectric region 252 thus remains along the bottom ofnon-gated trench 202.

In FIG. 6G, non-gated trench 202 is filled with a conductive material(e.g., highly doped p-type polysilicon) to form heavy body region 234. Asource interconnect layer 236 (e.g., comprising metal) is then formedover the structure to contact source regions 224 and heavy body regions234. In one embodiment, in forming heavy body region 234, the depositedconductive material is recessed into non-gated trench 202 to partiallyexpose sidewalls of source regions 224. This enables source interconnectlayer 230 to directly contact sidewalls of source regions 224 therebyreducing source contact resistance.

As can be seen, source regions 224 are self-aligned to the trenches. Inone embodiment wherein stripe shaped cell configuration is used, theprocess sequence depicted by FIGS. 6A-6G results in the formation ofcontinuous heavy body regions 234 which are also self-aligned. These andother self-aligned features of the resulting structure allow for a verytight cell pitch. Also, the masking steps typically required in formingeach of the source and heavy body regions are eliminated, thus reducingcost and minimizing process complexity.

In one embodiment, one non-gated trench is formed between every twogated trenches. In another embodiment, a larger ratio of non-gatedtrenches to gated trenches is used (e.g., two or more non-gated trenchesare formed between every two gated trenches) to reduce the gate-draincapacitance. In yet another embodiment, instead of forming the non-gatedand gated trenches at the same time, the non-gated trenches are formedat a different stage of the process than the gated trenches. While thisresults in additional processing steps, this embodiment providesflexibility in optimizing various features of the process and thestructure.

FIGS. 7A-7H are simplified cross section views illustrating anotherexemplary process flow for forming a shielded gate FET with self-alignednon-gated trenches incorporated between the gated trenches, according toan embodiment of the present invention. In FIG. 7A, gated trench 304 andnon-gated trench 302 are etched into n-type silicon region 300. In oneembodiment silicon region 300 comprises a highly doped n-type substrateand an n-type epitaxial layer over the substrate. In one variation ofthis embodiment, trenches 302 and 304 terminate within the epitaxiallayer, and in another variation, trenches 320 and 304 extend through theepitaxial layer and terminate within the substrate.

In FIG. 7A, a shield dielectric layer 306 (e.g., comprising oxide)extending over mesa surfaces 308 and lining the sidewalls and bottomsurfaces of trenches 302 and 304 is formed using known techniques. Apolysilicon layer is deposited and then recessed deep into trenches 302and 304 to thereby form shield electrodes 310 in trenches 302 and 304using conventional techniques. In FIG. 7B, a dielectric material 312(e.g., deposited film using SACVD) filling the trenches and extendingover the mesa regions is deposited, using conventional methods. In FIG.7C, a planarization process is carried out such that a top surface ofdielectric material 312 remaining in the trenches is substantiallyco-planar with mesa surfaces 308, using known techniques.

In FIG. 7D, using conventional methods, a masking layer (e.g.,photoresist) is deposited and patterned to form masking region 314covering non-gated trench 302, and then dielectric layer 306 anddielectric material 312 in gated trench 304 are recessed to apredetermined depth to thereby form inter-electrode dielectric 316 (IED)over shield electrode 310. In FIG. 7E, masking region 314 is removed anda gate dielectric layer 322 (e.g., comprising oxide) lining uppersidewalls of gated trench 304 and extending over mesa surfaces andnon-gated trench 302 is formed using conventional techniques. Apolysilicon layer is then deposited and recessed into gated trench 304to form recessed gate electrode 324 in gated trench 304. In FIG. 7F,conventional blanket body and source implantations are sequentiallycarried out in the active region of the device to form p-type bodyregions 328 in silicon region 300 and then form highly doped n-typesource regions 326 in body regions 328.

In FIG. 7G, using known techniques, a dielectric layer (e.g., comprisingBPSG) is formed over the structure and then patterned and etched to formdielectric cap 330 over gated trench 304. The same dielectric etch maybe used to recess dielectric materials 306 and 310 in non-gated trench302 sufficiently to partially expose sidewalls of body regions 328.Dielectric material 325 thus remains over shield electrode 310 innon-gated trench 302. Non-gated trench 302 is then filled with aconductive material (e.g., highly doped p-type polysilicon) to formheavy body region 332. A source interconnect layer 334 (e.g., comprisingmetal) is then formed over the structure to contact source regions 326and heavy body regions 332. In one embodiment, in forming heavy bodyregions 332, the deposited conductive material is recessed intonon-gated trench 302 to partially expose sidewalls of source regions326. This enables source interconnect layer 334 to directly contactsidewalls of source regions 326 thereby reducing source contactresistance.

As in the preceding embodiment, source regions 326 are self-aligned tothe trenches, and in the embodiment wherein stripe shaped cellconfiguration is used, the process sequence depicted by FIGS. 7A-7Hresults in the formation of continuous heavy body regions 332 which arealso self-aligned. These and other self-aligned features of theresulting structure allow for a very tight cell pitch. Also, the shieldelectrodes in the non-gated trenches allow the drift region resistivityto be lowered for the same breakdown voltage. Additionally, the maskingsteps typically required in forming each of the source and heavy bodyregions are eliminated, thus reducing cost and minimizing processcomplexity.

The shield electrodes in the gated and non-gated trenches may beelectrically connected to the source interconnect layer in a thirddimension or may be allowed to float. In one embodiment, one non-gatedtrench is formed between every two gated trenches. In anotherembodiment, a larger ratio of non-gated trenches to gated shieldedtrenches is used (e.g., two or more non-gated trenches are formedbetween every two gated trenches) to reduce the gate-drain capacitance.In yet another embodiment, instead of forming the non-gated and gatedtrenches at the same time, the non-gated trenches are formed at adifferent stage of the process than the gated trenches. While thisresults in additional processing steps, this embodiment providesflexibility in optimizing various features of the process and thestructure.

FIGS. 8A-8H are simplified cross section views illustrating yet anotherexemplary process flow for forming a shielded gate FET with self-alignednon-gated trenches incorporated between the gated trenches, according toanother embodiment of the present invention. In FIG. 8A, gated trench404 and non-gated trench 402 are etched into n-type silicon region 400.In one embodiment, silicon region 400 comprises a highly doped n-typesubstrate and an n-type epitaxial layer over the substrate. In onevariation of this embodiment, trenches 402 and 404 terminate within theepitaxial layer, and in another variation, trenches 402 and 404 extendthrough the epitaxial layer and terminate within the substrate.

In FIG. 8A, a shield dielectric layer 406 (e.g., comprising oxide)extending over mesa surfaces 414 and lining the sidewalls and bottomsurfaces of trenches 402 and 404 is formed using known techniques. Apolysilicon layer is deposited and etched back to slightly below the topsurface of shield dielectric layer 406, as shown. In FIG. 8B, usingconventional methods, a masking layer (e.g., photoresist) is depositedand patterned to form masking region 412 covering non-gated trench 402.In FIG. 8C, polysilicon 410 in gated trench 404 is then recessed deepinto the trench thereby forming shield electrode 410 in gated trench404. Masking region 412 is removed and then shield dielectric layer 406is etched back as shown. n FIG. 8D, a gate dielectric layer 420 (e.g.,comprising oxide) lining upper sidewalls of gated trench 404 andextending over shield electrode 410, the mesa surfaces and non-gatedtrench 402 is grown using conventional techniques. A polysilicon layeris then deposited and recessed into gated trench 404 to form recessedgate electrode 418 in gated trench 404. In FIG. 8E, conventional blanketbody and source implantations are carried out in the active region ofthe device to form p-type body regions 424 in silicon region 400 andthen form highly doped n-type source regions 422 in body regions 424.

In FIG. 8F, using known techniques, a dielectric layer (e.g., comprisingBPSG) is formed over the structure and then patterned and etched to formdielectric cap 426 over gated trench 404. The same dielectric etch maybe used to recess shield dielectric 406 in non-gated trench 402sufficiently to partially expose sidewalls of body regions 424. In FIG.8G, a conductive material (e.g., highly doped p-type polysilicon) isdeposited to fill non-gated trench 402 and then etched back, thusforming heavy body region 430 in non-gated trench 402. In FIG. 8H, asource interconnect layer 432 (e.g., comprising metal) is formed overthe structure to contact source regions 422 and heavy body regions 430.

As can be seen, source regions 422 are self-aligned to the trenches. Inthe embodiment wherein stripe shaped cell configuration is used, theprocess sequence depicted by FIGS. 8A-8H results in the formation ofcontinuous heavy body regions 430 which are also self-aligned. These andother self-aligned features of the resulting structure allow for a verytight cell pitch. Also, the shield electrodes in the non-gated trenchesallow the drift region resistivity to be lowered without degrading thebreakdown voltage. Additionally, the masking steps typically required informing each of the source and heavy body regions are eliminated, thusreducing cost and minimizing process complexity.

As can be seen, shield electrode 408 in non-gated trench 402 iselectrically connected to source interconnect 432 via heavy body region430. In one embodiment, one non-gated trench is formed between every twogated trenches. In another embodiment, a larger ratio of non-gatedtrenches to gated shielded trenches is used (e.g., two or more non-gatedtrenches are formed between every two gated trenches) to reduce thegate-drain capacitance. In yet another embodiment, instead of formingthe non-gated and gated trenches at the same time, the non-gatedtrenches are formed at a different stage of the process than the gatedtrenches. While this results in additional processing steps, thisembodiment provides flexibility in optimizing various features of theprocess and the structure.

FIG. 9 is a simplified cross section view of a shielded gate FET with anon-gated trench wherein the heavy body regions are formed in the bodyregions rather than inside the non-gated trench. The shielded gate FETstructure in FIG. 9 is similar to that in FIG. 7H except that heavy bodyregions 520 are formed in body regions 516, and the source interconnectlayer 518 extends into and fills an upper portion of non-gated trench502. Source interconnect layer electrically contacts source regions 514along the mesa surfaces and sidewalls of the source regions, andcontacts heavy body regions 520 along their sidewalls, as shown. Theremaining structural features of the FET in FIG. 9 are similar to thosein FIG. 7H and thus will not be described.

The process flow for forming the FET structure in FIG. 9 is similar tothat depicted by FIGS. 7A-7H except for the following changes. In FIG.7G, after recessing dielectric materials 306 and 310 in non-gated trench302 whereby sidewalls of body regions 328 are partially exposed, atwo-pass angled implant of p-type dopants into exposed sidewalls ofnon-gated trench 302 is carried out to form heavy body regions 520 (FIG.9) in the body regions. In one embodiment, no mask is used in carryingout the two-pass angled implant, and the heavy body implant dose isselected to be lower than that for the source regions so that theeffective doping concentration of the source regions in the vicinity ofthe non-gated trench is not impacted by the heavy body implant in anysignificant way.

In FIG. 7H, upon depositing the source interconnect layer over thestructure, the source interconnect layer fills the non-gated trench thuselectrically contacting the heavy body regions and the source regionsalong their sidewalls as shown in FIG. 9. The embodiment in FIG. 9 hasthe same features and advantages as the embodiments depicted by FIG. 7Hdescribed above. Also, the alternate variations and embodiments of theFIGS. 7A-7H embodiment described above also apply to the FIG. 9 FETstructure.

FIG. 10 is a simplified cross section view of a trench gate FET with anon-gated trench wherein the heavy body regions are formed in the bodyregions rather than inside the non-gated trench. The trench gate FETstructure in FIG. 10 is similar to that in FIG. 6G except that heavybody regions 620 are formed in body regions 618, and the sourceinterconnect layer 622 extends into and fills an upper portion ofnon-gated trench 602. Source interconnect layer electrically contactssource regions 514 along the mesa surfaces and sidewalls of the sourceregions, and contacts heavy body regions 520 along their sidewalls, asshown. The remaining structural features of the FET in FIG. 10 aresimilar to those in FIG. 6G and thus will not be described.

The process flow for forming the FET structure in FIG. 10 is similar tothat depicted by FIGS. 6A-6G except for the following changes. In FIG.6F, after recessing dielectric materials 206 and 210 in non-gated trench202 whereby sidewalls of body regions 226 are partially exposed, atwo-pass angled implant of p-type dopants into exposed sidewalls ofnon-gated trench 202 is carried out to form heavy body regions 620 (FIG.10) in the body regions. In one embodiment, no mask is used in carryingout the two-pass angled implant, and the heavy body implant dose isselected to be lower than that for the source regions so that theeffective doping concentration of the source regions in the vicinity ofthe non-gated trench is not impacted by the heavy body implant in anysignificant way.

In FIG. 6G, upon depositing the source interconnect layer over thestructure, the source interconnect layer fills the non-gated trench thuselectrically contacting the heavy body regions and the source regionsalong their sidewalls as shown in FIG. 10. The embodiment in FIG. 10 hasthe same features and advantages as the embodiments depicted by FIG. 6Gdescribed above. Also, the alternate variations and embodiments of theFIGS. 6A-6G embodiment described above also apply to the FIG. 10 FETstructure.

The various structures and methods of the present invention may becombined with one or more of a number of charge balance and shieldedgate techniques (e.g. those in FIGS. 2A-2B, 3A-3B, 4A-4E, 5B-5C, 6-8,9A-9C, 10-24, as well as other device structures and manufacturingprocesses disclosed in the commonly assigned application Ser. No.11/026,276, filed Dec. 29, 2004, and incorporated herein by reference inits entirety, to achieve an even lower on-resistance, higher blockingcapability and higher efficiency, among other advantages and features.Additionally, one or more of the various shielded gate structures (e.g.,those in FIGS. 4-7) and methods for forming them disclosed in theabove-referenced commonly assigned U.S. patent application Ser. No.11/441,386, filed on May 24, 2006, can be advantageously combined withone or more of the charge balance techniques (e.g., those in FIGS.3A-3E, 4, 5A-5B, 7A-7H, 8A-8H, 9-10) disclosed herein to obtain chargebalance shielded gate devices with optimized performance and structuralcharacteristics.

The cross-section views of the different embodiments described hereinmay not be to scale, and as such are not intended to limit the possiblevariations in the layout design of the corresponding structures.

Although a number of specific embodiments are shown and described above,embodiments of the invention are not limited thereto. For example, whilethe various embodiments described above are implemented in conventionalsilicon, these embodiments and their obvious variants can also beimplemented in silicon carbide, gallium arsenide, gallium nitride, orother semiconductor materials. As another example, while the aboveembodiments are described in the context of n-channel transistors,p-channel counterpart transistors can be formed by simply reversing theconductivity type of the various regions. Also, the various transistorsdescribed herein can be formed in open or closed cell configurations,including hexagonal, oval or square shaped cells. Further, theembodiments of the present invention are not limited to MOSFETs. Forexample, the modifications necessary to form IGBT counterparts of theabove-described MOSFETs would be obvious to one skilled in this art inview of this disclosure. Additionally, while some of the embodimentsdescribed herein are particularly useful for low voltage applications,the process flows and structures described herein may be modified by oneskilled in the art in view of this disclosure to form transistors thatare more suitable for high voltage applications and have many of thesame advantages and features of the present invention. Moreover, thefeatures of one or more embodiments of the invention may be combinedwith one or more features of other embodiments of the invention withoutdeparting from the scope of the invention.

Therefore, the scope of the present invention should be determined notwith reference to the above description but should, instead, bedetermined with reference to the appended claim, along with their fullscope of equivalents.

What is claimed is:
 1. A method of forming a FET comprising: providing asemiconductor region of a first conductivity type with an epitaxiallayer of a second conductivity extending over the semiconductor region;performing a first silicon etch to form an upper trench portionextending into and terminating within the epitaxial layer; formingprotective material extending along sidewalls of the upper trenchportion and over mesa regions adjacent the upper trench portion but notalong a bottom surface of the upper trench portion; performing a secondsilicon etch to form a lower trench portion extending from the bottomsurface of the upper trench portion through the epitaxial layer andterminating within the semiconductor region, the lower trench portionbeing narrower than the upper trench portion; and performing a two-passangled implant of dopants of the first conductivity type to form asilicon region of first conductivity type along sidewalls of the lowertrench portion, the protective material blocking the implant dopantsfrom entering the sidewalls of the upper trench portion and the mesaregion adjacent the upper trench portion.
 2. The method of claim 1wherein the step of forming protective material comprises: forming adielectric layer extending along sidewalls and bottom surface of theupper trench portion and over mesa regions adjacent the upper trenchportion; forming dielectric spacers along sidewalls of the upper trenchportion; and removing a portion of the dielectric layer extending alongthe bottom surface of the upper trench portion so as to expose a surfaceof the epitaxial layer.
 3. The method of claim 2 wherein the dielectriclayer comprises oxide, and the dielectric spacers comprise nitride. 4.The method of claim 2 wherein the dielectric layer is formed such that aportion of the dielectric layer extending over the mesa regions isthicker than a portion of the dielectric layer extending inside thetrench.
 5. The method of claim 1 further comprising: forming a shieldelectrode in the lower trench portion, the shield electrode beinginsulated from the epitaxial layer and the semiconductor region by ashield dielectric; forming a gate dielectric layer along sidewalls ofthe upper trench portion; and forming a gate electrode in the uppertrench portion over but insulated from the shield electrode.
 6. Themethod of claim 1 further comprising: forming source regions of thefirst conductivity type in the epitaxial layer; forming heavy bodyregions of the second conductivity type in the epitaxial layer; andforming a source interconnect layer contacting the source regions andthe heavy body regions.
 7. The method of claim 1 wherein the firstconductivity type is n-type and the second conductivity type is p-type.8. The method of claim 1 wherein the semiconductor region is a highlydoped substrate.
 9. A method of forming a MOSFET comprising: providing asubstrate of a first conductivity type with an epitaxial layer of asecond conductivity extending over the substrate; performing a firstsilicon etch to form an upper trench portion extending into andterminating within the epitaxial layer; forming protective materialextending along sidewalls of the upper trench portion and over mesaregions adjacent the upper trench portion but not along a bottom surfaceof the upper trench portion; performing a second silicon etch to form alower trench portion extending from the bottom surface of the uppertrench portion through the epitaxial layer and terminating within thesubstrate, the lower trench portion being narrower than the upper trenchportion; performing a two-pass angled implant of dopants of the firstconductivity type to form a silicon region of first conductivity typealong sidewalls of the lower trench portion, the protective materialblocking the implant dopants from entering the sidewalls of the uppertrench portion and the mesa region adjacent the upper trench portion;forming a shield dielectric lining sidewalls and bottom surface of thelower trench portion; forming a shield electrode in the lower trenchportion; forming a gate dielectric layer along sidewalls of the uppertrench portion; and forming a gate electrode in the upper trench portionover but insulated from the shield electrode.
 10. The method of claim 9wherein the step of forming protective material comprises: forming aprotective dielectric layer extending along sidewalls and bottom surfaceof the upper trench portion and over mesa regions adjacent the uppertrench portion; forming dielectric spacers along sidewalls of the uppertrench portion; and removing a portion of the protective dielectriclayer extending along the bottom surface of the upper trench portion soas to expose a surface of the epitaxial layer.
 11. The method of claim10 wherein the protective dielectric layer comprises oxide, and thedielectric spacers comprise nitride.
 12. The method of claim 10 whereinthe protective dielectric layer is formed such that a portion of theprotective dielectric layer extending over the mesa regions is thickerthan a portion of the protective dielectric layer extending inside thetrench.
 13. The method of claim 9 further comprising: forming sourceregions of the first conductivity type in the epitaxial layer; formingheavy body regions of the second conductivity type in the epitaxiallayer; and forming a source interconnect layer contacting the sourceregions and the heavy body regions.
 14. The method of claim 9 whereinthe first conductivity type is n-type and the second conductivity typeis p-type.
 15. A MOSFET comprising: a trench having a lower portion andan upper portion, the lower portion being narrower than the upperportion, the trench extending into a semiconductor region; a shieldelectrode in the lower portion of the trench, the shield electrode beinginsulated from the semiconductor region by a shield dielectric; a gateelectrode in the upper portion of the trench, the gate electrode beingover but insulated from the shield electrode; wherein the semiconductorregion comprises: a substrate of a first conductivity type; a firstsilicon region of a second conductivity type over the substrate, thefirst silicon region having a first portion extending to a depthintermediate a top surface and a bottom surface of the gate electrode,the first silicon region having a second portion extending to a depthintermediate a top surface and a bottom surface of the shield electrode;a second silicon region of the first conductivity type between the lowertrench portion and the second portion of the first silicon region, thesecond silicon region having a laterally-graded doping concentrationdecreasing in a direction away from the sidewalls of the lower trenchportion; and a source region of the first conductivity type in the firstsilicon region, the source region being adjacent the upper trenchportion.
 16. The MOSFET of claim 15 wherein the lower trench portionterminates within the substrate.
 17. The MOSFET of claim 15 wherein thefirst portion of the first silicon region includes avertically-extending channel region defined by a spacing between thesource region and the second silicon region.
 18. The MOSFET of claim 15further comprising: a heaving body region of the second conductivitytype in the first silicon region; and a source interconnect layerelectrically contacting the source region and the heavy body region butbeing insulated from the gate electrode.